Optical spectroscopy of the density of gap states in ETP-deposited a-Si:H

The distribution and density of localized states in the band gap of hydrogenated amorphous silicon, as deposited by the expanding thermal plasma technique, were studied by means of a combined use of the constant photocurrent method (CPM), photothermal deflection spectroscopy (PDS) and time-of-flight (TOF) transient photoconductivity, as well as by standard optical transmission measurements. Tauc optical gap and Urbach tail energy parameters were determined, together with estimates for the density of defect states in the gap, from the CPM and PDS absorption spectra. Analysis of the post-transit TOF signals gives a further set of independent values for the gap state density.